The probability of achieving viral replication suppression during the treatment with DRV has been related to both the extent of viral resistance to DRV (inhibitory concentration 50%, IC50) and the drug concentration. Moreover, the DRV virtual inhibitory quotient (vIQ) has been related significantly with the virological response to DRV treatment. So patients with a DRV vIQ >= 1,5 had a 8-times higher probability of having viral load < 50 copies/mL after 24 weeks of treatment than those having a vIQ < 1,5.